Lecture 6: Matter and Energy II

Water on Earth

  • Water molecules have chemical characteristics that support life
  • They are polar which facilitates bonding with other chemicals
  • "universal solvent"
  • Liquid over a wide range of temperatures
  • Stabilizes systems against change
  • Water ­based processes occur in a variety of conditions
  • Strong cohesion
  • Facilitates the transport of chemicals
  • High heat capacity
  • Transparent to light
  • Photosynthesis is possible
  • Solid less dense than liquid
  • Floating ice insulates water

Earth's Early Atmosphere

  • Primitive, inhospitable and very different from the one we have now
  • Role of volcanic degassing
  • Reducing atmosphere­ \rightarrow no "free" oxygen (oxidizing atmosphere)
  • Rich in CO2, CO, SO2, CH4 etc.
  • Hot, lots of volcanism
  • Dense­, high atmospheric pressure


Dynamic Equilibrium

  • The earth's atmosphere-­hydrosphere­-geosphere interacted dynamically
  • Surface cooled enough to sustain liquid water
  • Oceans began to accumulate (~4.4 Ga)
  • Still hot
  • Water precipitated and re-­evaporated
  • Water + atmospheric gases \rightarrow acids
  • Resulted in weathering of minerals
  • Cyclical chemical and physical interactions:
  • Hydrosphere + atmosphere + geosphere


Proposed Hypotheses to Explain Life's Origin

Heterotrophic Hypothesis

  • "Primordial Soup"
  • Life originated from inorganic chemicals in the ocean

Panspermia Hypothesis

  • First life forms used organic compounds for energy
  • "Seeds" from space
  • Microbes delivered to Earth through meteorites
  • Organic compounds found in meteorites

Chemoautotrophic hypothesis

  • Ocean depths
  • Life originated at deep-sea hydrothermal vents
  • First organisms were chemoautotrophs


Fossil Evolution

Fossils - Remains of a long-­dead organism, preserved in stone

Fossil Record - Gives info about the history of past life

  • New life forms have evolved throughout Earth history

Macro-Fossils - Chemical evidence that "life was here"

  • Not all fossils are macro-fossils


Central role of life in the evolution of Earth systems

  • Early life forms:
  • Chemo synthetic, thermophilic bacteria
  • Photosynthetic cyanobacteria ("blue-­green algae")
  • Life's two main influences on the chemistry of atmosphere
  • Buildup of oxygen through stromatolites and aerobic which are Cyanobacteria
  • Removal of carbon dioxide through early life, organic matter deposited as sediment and carbon cycle

Unique Characteristics of Earth

  • Specific chemical composition of atmosphere
  • Nitrogen and oxygen rich
  • Existence of water in three states near surface
  • Solid, liquid, vapour
  • Dynamic nature of rock weathering
  • Soil \rightarrow organic + inorganic components
  • Dynamic nature of lithosphere
  • Plate tectonics


Conclusions

  • Earth has several characteristics that make it unique amongst planets
  • Earth has the capacity of supporting life
  • Early Earth was hostile to life
  • The atmosphere, hydrosphere, and geosphere have evolved chemically in dynamic equilibrium
  • Life played a central role in the chemical evolution of Earth's environmental systems


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